Heretofore, in general, as a device for performing an auto focus detection (AF: auto focus) of a camera, the following camera has been known. That is, in this auto focus device, light flux from an object having passed through different exit pupil areas of an image pickup lens is focused onto a pair of line sensors. The object image is photoelectrically converted and then a pair of image signals are outputted. From a displacement amount of the relative position of this pair of image signals, a defocus amount is detected, and based on this defocus amount, the driving of the image pickup lens is performed (Patent Document 1: Japanese Patent Application Laid-Open No. H09-054242). Hereinafter, such focus detection system is referred to as a phase difference detection system.
Further, in a digital camera using an electronic image pickup sensor, an auto focus device has been also well-known, in which, while a focus position is changed by driving a photographic lens, the estimation of the contrast of an object obtained by the image pickup sensor is performed, thereby detecting an in-focus position (Patent Document 2: Japanese Patent Application Laid-Open No. 2001-004914). Hereinafter, this focus detection system is referred to as a contrast detection system.
Further, for the electronic image pickup apparatus comprising both of the phase different detection system and the contrast detection system described above, a device has been also known, in which a difference between the in-focus position detected by the phase difference detection system and the in-focus position detected by the contrast detection system is stored in advance as a correction value, and at the normal photographing time, the photographic lens is driven in accordance with the in-focus position detected by the phase difference detection system and the stored correction value (Patent Document 3 Japanese Patent Application Laid-Open No. JP 2000-292684).
Since the phase difference detection system disclosed in the Patent Document 1 can detect a defocus amount in a range from a certain focus position to an in-focus position, a time required for the in-focus detection is short. However, since an object light path obtained by the phase difference detection system is different from a object light path in the case of the actual photographing, there arises an error between the in-focus position detected by the phase difference detection system and the in-focus position in an image pickup plane.
Further, in the contrast detection system disclosed in the Patent Document 2, since the focus detection is performed on the object light path when photographing, the in-focus position on the image pickup plane can be accurately detected. However, it is necessary to perform a contrast estimation calculation while changing the in-focus position of the image pickup lens, and this arises a problem that it takes time until the in-focus position is reached.
Further, in the electronic image pickup apparatus disclosed in the Patent Document 3, there arises a problem that, though the in-focus position displacement detected by the phase difference detection system can be accurately corrected using the correction value, an appropriate correction value cannot be detected depending on the condition of the object when detecting the correction value. For example, in case the contrast of the object is low, there arises a big error in each of the detection result of the phase difference detection system and the detection result of the contrast detection system. In case of the infinite focusing distance where the distance to the object is greatly away from the normal distance, and the closest focusing distance, there arises an error in the detection result due to the influence such as the stopping accuracy of the photographic lens and the like. In addition, since the in-focus position is displaced due to chromatic aberration of the photographic lens, it is not possible to find an accurate correction value. In this manner, the correction value changes according to measurement condition, and thus, it is necessary to restore the measurement condition to a predetermined measurement condition to detect an intrinsically accurate correction value. However, while it is easy for the plant and the like to create the predetermined measurement condition, it is difficult for the general user to create the predetermined measurement condition.